Deflection circuits coupled via a filter

ABSTRACT

A secondary winding of a horizontal flyback transformer of a horizontal deflection circuit develops a horizontal retrace pulse voltage. A secondary winding of a second transformer is coupled in series with a vertical deflection coil of a vertical deflection circuit. An R-C filter is coupled between the secondary winding of the flyback transformer and a primary winding of the second transformer. Horizontal parallelogram errors are corrected by a horizontal rate current injected in a current path of the vertical deflection coils. The R-C filter prevents the vertical deflection current from being parasiticaly coupled to the horizontal deflection circuit.

This Application claims Benefit of provisional Application Ser. No.60/115,709 filed Jan. 12, 1999.

FIELD OF THE INVENTION

The invention relates to raster correction circuits of a video display.

BACKGROUND

In a cathode ray tube (CRT) of a video display, a raster is formed bydeflecting an electron beam across a phosphor screen. Each electron beamis deflected in a horizontal direction by a magnetic field produced byin a horizontal deflection coil by a horizontal-rate sawtooth current.Likewise, the electron beam is simultaneously deflected in a verticaldirection by a magnetic field produced by a vertical deflection coil bya vertical-rate sawtooth current. The result is a negatively-sloped, or“downhill”, scan line as the electron beam is deflected from left toright to form the CRT's raster. In a typical cathode ray tube used in acolor television receiver and having, for example, a screen width ofapproximately 723 mm and a screen height of approximately 538 mm, ahorizontal scan line may drop a distance of approximately 2.4 mm from aperfectly horizontal position in one field. This downhill scan effectintroduces both orthogonality and parallelogram errors into the raster.

In a perfectly rectangular raster, horizontal and vertical center linesare orthogonal, or perpendicular, to one another. Downhill scanning doesnot produce a perfectly rectangular raster and hence results in anon-orthogonal relationship between the horizontal and vertical centerlines of the raster. Orthogonality error is a quantitative measure,expressed in units of radians or degrees, of the extent to which thehorizontal and vertical center lines of a raster depart fromorthogonality. The orthogonality error may be magnified at the left andright edges of the raster because the deflection sensitivity increasesnear the edges of the raster. As a result, the edges of the raster maytilt such that the raster has a generally parallelogram shape. errors ina raster can be obtained by providing a horizontal-rate modulation of avertical deflection current for substantially offsetting the downhillscan effect caused by vertical deflection of the electron beam. In oneof the circuits shown

Elimination of both orthogonality and parallelogram errors in a rastercan be obtained by providing a horizontal-rate modulation of a verticaldeflection current for substantially offsetting the downhill scan effectcaused by vertical deflection of the electron beam. A winding of ahorizontal flyback transformer can be used to apply a horizontal retracepulse voltage to a primary winding of a transformer. A secondary windingof the transformer can be coupled to a vertical deflection winding forproviding a small horizontal rate sawtooth current to be superimposed ona vertical deflection current.

Coupling back of the vertical current to the horizontal deflectioncircuit is reduced by the relatively large leakage of the transformer.Nevertheless, the residual vertical rate current, during verticalretrace, can still produce a disturbance at the top of the screen,immediately after vertical retrace. It may be desirable to furtherreduce the coupling back of the vertical current to the horizontaldeflection circuit.

In carrying out an inventive feature, an R-C filter is interposed in acurrent path between the transformers. The R-C filter attenuates thecoupled back vertical deflection current. Thereby, the addition of theR-C coupling filter prevents the vertical deflection current fromaffecting the horizontal deflection circuit.

SUMMARY OF THE INVENTION

A video display deflection apparatus, embodying an inventive feature,includes a first deflection circuit for generating a first deflectioncurrent at a first deflection frequency in a first deflection winding tovary a position of an electron beam in a first direction. A seconddeflection circuit is used for generating a second deflection current ina second deflection winding at a second deflection frequency to vary theposition of the electron beam in a second direction. A filter couplesthe second deflection circuit to the first deflection winding togenerate a corrective current in a current path formed by the firstdeflection winding at a frequency related to the second deflectionfrequency for providing raster error correction. The filtersignificantly attenuates parasitic signal coupling in an oppositedirection, from the first deflection circuit to the second deflectioncircuit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an arrangement for correcting orthogonality andparallelogram errors in a raster, including a filter, in accordance withan inventive feature;

FIGS. 2a and 2 b illustrate waveforms useful for explaining theoperation of the deflection system shown in FIG. 1, when the filter isemployed; and

FIGS. 3a and 3 b illustrate waveforms useful for explaining theoperation of the deflection system shown in FIG. 1, when the filter isremoved.

DETAILED DESCRIPTION

A deflection system 100 of FIG. 1 provides deflection for a cathode raytube, not shown, of a television receiver or a video display terminal. AB⁺ voltage is coupled to a conventional horizontal deflection circuit 20through a primary winding L_(PRI) of a flyback transformer IHVT. Adamper current I_(D) flows through a damper diode D1 to deflect anelectron beam from a left edge of a raster to a center of the raster. Ahorizontal output transistor Q1 conducts a current I_(HOT) to deflectthe electron beam from the center of the raster to a right edge of theraster. A horizontal deflection current I_(H) flowing through ahorizontal deflection winding L_(H) may have a peak-to-peak amplitude ofapproximately 12 A. A trace capacitor C_(S), coupled in series withdeflection winding L_(H) provides S-correction for the horizontaldeflection current I^(H).

A secondary winding L_(SEC) of flyback transformer IHVT is coupled viaan R-C filter 40, embodying an inventive feature, to a primary winding42 of a raster correction transformer 41. Transformer 41 has a secondarywinding 43. Transformer 41 is wound on a ferrite slug core 1″long×0.399″ diameter. Winding 43 has N_(S)=60 turns, 5-strand LitzAWG#30 wire, and winding 42 has N_(P)=180 turns, AWG#29 wire.

A horizontal-rate retrace pulse, not shown, produced in a conventionalmanner in deflection circuit 20, is transformer-coupled to secondarywinding L_(SEC) of transformer IHVT to develop a horizontal-rate retracepulse 12. Retrace pulse 12 is coupled via R-C filter 40, embodying aninventive feature, to winding 42 of transformer 41. Transformer 41 stepsdown a significant portion of horizontal-rate pulse 12 coupled throughR-C filter 40 and developed in winding 42 according to transformer 41turns ratio. Raster correction transformer 41 develops a stepped-downhorizontal-rate pulse waveform 11 with a peak-to-peak voltage ofapproximately 50 Vpp across secondary winding 43. Similarly, ahorizontal raster correction current I_(CORR) is induced in secondarywinding 43.

A direct current (DC) coupled vertical deflection circuit 60 includes aconventional vertical-rate sawtooth generator 61 that provides avertical-rate sawtooth waveform to a non-inverting input of aconventional vertical output amplifier 62. Vertical output amplifier 62may include a push-pull transistor output stage, not shown. Verticaloutput amplifier 62 drives a vertical deflection windings L_(V1) and avertical deflection windings L_(V2), coupled in series, with avertical-rate sawtooth current I_(V). Current I_(V) may have apeak-to-peak amplitude of approximately A. (2.6 App)

Vertical deflection windings L_(V1) and L_(V2) are also coupled inseries with winding 43 of transformer 41 and with resistor R4.Current-sense resistor R4 generates a feedback voltage at an invertinginput of vertical output amplifier 62 responsive to the verticaldeflection current I_(V). Except for the modulation provided by rastercorrection current I_(CORR) induced in secondary winding 43, verticaldeflection circuit 60 generates current I_(V) in a conventional manner.Horizontal rate raster correction current I_(CORR) flows through bothvertical deflection windings L_(V1) and L_(V2) to produce a magneticfield which opposes the aforementioned downhill scan effect.

For explanation purposes, assume that filter 40 is not used. Instead,assume that winding L_(SEC) of high-voltage transformer IHVT is coupleddirectly in parallel with winding 42 of transformer 41, as shown by ajumper conductor 40 a.

Vertical deflection current I_(V) flows through secondary winding 43 oftransformer 41. During vertical retrace, a vertical pulse voltage V_(V)of FIG. 3b, developed across windings L_(V1) and L_(V2) of FIG. 1,produces a vertical rate current component in a current 142 of winding42 of transformer 41. Vertical rate modulation of current 142 of FIG.3a, during the retrace portion of vertical pulse voltage V_(V) of FIG.3b, shifts the average value of current 142 in a vertical rate. Similarsymbols and numerals in FIGS. 1, 3 a and 3 b indicate similar items orfunctions.

The vertical rate current component in current 142 of FIG. 1 may becoupled back to horizontal deflection circuit 20 via transformer IHVTand, disadvantageously, may initiate ringing in horizontal deflectionwinding L_(H). A resulting width disturbance can become visible on thedisplay screen, not shown.

In carrying out an inventive feature, the coupling back from thevertical to the horizontal is reduced or eliminated by the addition ofR-C filter 40 between winding L_(SEC) of transformer IHVT and winding 42of transformer 41. This situation is demonstrated, when jumper conductor40 a in FIG. 1 is removed and filter 40 is interposed. Capacitor C offilter 40 forms a low impedance for horizontal rate current component ofcurrent 142. Therefore, Capacitor C of filter 40 does not attenuate thehorizontal rate current component of current 142. On the other hand, forthe vertical rate current component of current 142, capacitor C forms ahigh impedance and acts as an attenuator. Thereby, coupling back, isadvantageously, attenuated significantly.

The waveform of primary current 142 when R-C filter 40 is in circuit isshown in FIG. 2a. In contrast to the waveform in FIG. 3a, verticaldeflection current I_(V) of FIG. 2b, during vertical retrace,advantageously, does not produce any significant vertical rate currentcomponent in current 142 of FIG. 2a. Similar symbols and numerals inFIGS. 1, 3 a, 3 b, 2 a and 2 b indicate similar items or functions. Theelimination of the parasitic, back coupling effect in current 142 ofFIG. 2a from current I_(V) of FIG. 2b, advantageously, eliminates thewidth artifact at the start of vertical scan.

A damping circuit 60 is formed by a resistor R1 and a capacitor C1,coupled in series. Circuit 60, is coupled between a center tap 21,approximately in the midpoint of vertical deflection windings L_(V1),and a center tap 21, approximately, in the midpoint of verticaldeflection windings L_(V2).

The effectiveness of the injection of parallelogram/orthogonality errorcorrection current ICORR by winding 43 at an end terminal 43 a of thevertical deflection windings Lv_(V1) and L_(V2), that is remote fromamplifier 62, is facilitated by installing damping circuit 60 formed byresistor R1 and capacitor C1. Damping circuit 60 increases thesensitivity of windings L_(V1) and L_(V2) to correction currentI_(CORR). Consequently, single ended drive is sufficient.

What is claimed is:
 1. A video display deflection apparatus, comprising:a vertical deflection circuit for generating a vertical deflectioncurrent at a vertical deflection frequency in a vertical deflectionwinding to vary a position of an electron beam in a vertical direction;a horizontal deflection circuit for generating a horizontal deflectioncurrent in a horizontal deflection winding at a horizontal deflectionfrequency to vary the position of said electron beam in a horizontaldirection; and a filter for coupling said horizontal deflection circuitto said vertical deflection winding to generate a corrective current ina current path formed by said vertical deflection winding at a frequencyrelated to said horizontal deflection frequency for providing rastererror correction, said filter significantly attenuating parasitic signalcoupling in an opposite direction, from said vertical deflection circuitto said horizontal deflection circuit.
 2. The deflection apparatusaccording to claim 1, wherein said corrective current substantiallyreduces a downward slope imparted to said electron beam as said electronbeam is deflected between first and second lateral edges of said raster.3. The deflection apparatus according to claim 1, wherein saidcorrective current corrects at least one of parallelogram andorthogonality errors.
 4. The deflection circuit of claim 1, wherein saidcorrective current has a horizontal scanning rate.
 5. The deflectioncircuit of claim 1, wherein said vertical deflection circuit providesvertical deflection and includes a winding of a first transformer andsaid horizontal deflection circuit includes a winding of a horizontalflyback transformer and wherein said filter is coupled in a current pathbetween said transformers.
 6. The deflection apparatus according toclaim 1 wherein said filter comprises a capacitor having a low impedanceat said horizontal deflection frequency for coupling a horizontal ratesignal from said horizontal to said vertical deflection circuit withoutsignificant attenuation and a high impedance at said vertical deflectionfrequency for attenuating a vertical rate signal of said verticaldeflection circuit.